l2-data link control

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CS2105: Introduction to Computer Networks

Anand BhojanCOM2-02-46, School of [email protected]: 651-67351

Anand BhojanCOM2-02-46, School of Computingbanand@

comp.nus.edu.sgph: 651-67351

L2: Principles of Reliable Data

Transfer (Data Link Control)

FramingFlow and Error Control Concepts

Stop-and-Wait ARQ Protocol

Go-Back-N Protocol

Selective-Repeat Protocol


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Data Link Layer

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Data Transfer between two directly connected nodes. (Hop-to-Hop communication, Point-to-point communication)

Data link layer divided into two functionality-oriented sublayers

application

transport

network

link

physical

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Data Link Control

Multiple Access Resolution

Data Link Layer


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Data Link Layer - Framing

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Bits must be framedinto discernible blocks ofinformation.

Message/Data is broken up into recoverablechunks that can easily be checked for corruption.

Frames have Source and Destination addresses Separates/identifies frames from different sources

at the destination.

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Data Link Control Protocols - Simple Protocol

Assumes noiseless channel and infinite buffer

Note:- Shown as Link layer Protocol. It is also applicable for Transport Layer.

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Flow control - concept

What happens if the channel is noiseless but thereceiver has finite buffer?

Receiver processes data at its own speed. Sender cannot overwhelm the receiver with large amount of data.

Flow Control is required! How?

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Stop-and-Wait Protocol

What happens if the channel is noiseless but thereceiver has finite buffer?

Flow control refers

to a set of

procedures used torestrict the

amount of data

that a sender can

send before waiting

for an

acknowledgment(ACK).

Flow Control is required

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Error Control - concept

Channel is noisy (errors, losses anddelay) with finite buffer.

Error Control is required! How?

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Most Common Error Control - concept

Generally frames have some error detectionmechanism (will be discussed in next Lecture)

If received frame is in error, receiver requests forretransmission Automatic Repeat Request (ARQ)

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Stop and Wait ARQ - Design

Case 1: Lost or Corrupt Frame. How to find and retransmit?

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Case 2: Lost or Corrupt ACK.

Side effect?

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Case 3: Delayed Frame/ACK

Duplicate ACK

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Stop and Wait ARQSender maintainsa

timerfor each

transmittedframe

maintains acopy of eachtransmittedframe and

retransmitswhen required

numbers theframes

Numbers theACKs (nextexpectedframenumber)

Summary

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Performance of Stop and Wait ARQ

Stop-and-wait ARQ works, but performance stinks

example: 1 Gbps link, 15 ms end-to-end Prop Delayfrom east coast to west coast of US (RTT=30 ms;approx speed of light), 1KB packet:

Ttransmit

= 8kb/pkt

10**9 b/sec = 8 microsec

U sender: utilization of sender or utilization of the channel(time sender busy sending / total time)

L (packet length in bits)

R (transmission rate, bps) =

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first packet bit transmitted, t = 0

sender receiver

RTT

last packet bit transmitted, t = L / R

first packet bit arrives

last packet bit arrives, send ACK

ACK arrives, send nextpacket, t = RTT + L / R

Performance of Stop and Wait ARQ

As BDP increases we need to transmit more bits or

frames (before waiting for ACK) to improve the utilisation

Effect of increasing Distance (or RTT)? Effect of increasing digital BW? == BDP

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Pipelined protocols

Pipelining: sender allows multiple, in-flight, yet-

to-be-acknowledged pkts

range of sequence numbers must be increasedbuffering at sender and/or receiver

Two generic forms of pipelined protocols:go-

Back-N, selective repeat

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Analogy:

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Current: An Analogy: Talking on a

Cell Phone.

Bob says a sentencesand waits for ACK(uh huh) before

saying next sentence.

Pipe-lined: An Analogy: Talking on a

Cell Phone.

Bob is allowed to say Nsentences and waits forACK (uh huh) before

saying next sentence.

Questions: How much is N?

Can N be infinite? Cumulative ACK

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Go-back-N ARQ (GBN)

How many in-flight packets? N Packets (sliding window of sizeN on sequence numbers) --- [Flow Control]

Out-of-order packets are discarded. Because of this, Receivewindow size of GBN is 1 and acknowledgement in GBN is

cumulative. Packet losses uses timer; retransmit on timeout

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Some examples follows

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Send 1 PacketEg 1:

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Receive ACK 4Eg 2:

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Receive ACK 6Eg 3: Cumulative ACK

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Full WindowEg 4:

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Empty WindowEg 5:

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Go-back-N ARQ

Why it is called Go-back-N? When a timer expires all frames starting from the frame in error are

retransmitted.

Example:Frame 3

Frame 4Frame 5

Frame 6

Frame 3

Frame 4

Frame 5

Frame 6

Timer ExpiredGo back and retransmit all

frames starting from frame 3!

Sender Receiver

Stop-and-Wait ARQ is a special case

of Go-Back-N ARQ in which the size

of the send window is 1.

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GBN eg1: Lost ACKs

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The size of the receive window

is always 1 in Go-back-N

GBN eg2: Lost Frame

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Go-back-N ARQ window SIZE If the size of sequence number field is m bits,

then we can transmit a maximum of2m

-1 frames. Example: 2 bit sequence number

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Selective-Repeat ARQ (SR)

GBN leads to unnecessary retransmission since outof order packets are discarded.

SR sender retransmits only the lost (or invalid; error)packets. (Improvement over GBN).

Requirements at receiver side Receiver should buffer out of order packets. Receiver needs a

receiver-window of size of size equal to send window.

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Selective-Repeat ARQ (SR)

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SR eg1: Lost Frame

Instead NAK one can use Duplicate ACK to tell the expected frame to the sender

What happens if NAK 1 is lost?

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Selective Repeat ARQ

In Selective Repeat ARQ, the size of the sender andreceiver window must beat most one-half of 2

m

.

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Piggybacking

Communication is generallyfull-duplex

acknowledgement can bepiggybacked on data in

the opposite direction.

Sender Receiver

Frame0

Frame0

Frame 1, ACK 2

Frame2,ACK2

ACK1

Frame1ACK1

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Summary

Flow control and Error controlSimple Protocol (No flow and error control)Stop and Wait (for flow control)Stop and Wait ARQ (adds error control to SW)Pipelined Protocols (improves efficiency of SW-ARQ)GBN : receive window size = 1SR : receiver window = send window size

READ THE TEXT BOOK.

(Slides alone are not enough)

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